Confused by HERS rating
STCook
| Posted in Energy Efficiency and Durability on
I recently finished a new home build, as part of that build a HERS rating was done. When I got the HERS report I noted several errors regarding insulation levels, wall construction, Makeup air, HVAC equipment etc. I pointed these errors out and requested an updated report. When I got the new report, which showed a reduction in MMBtu of 36%, but the HERS number INCREASED?
I pointed this out as I found it very non intuitive that the same house, now shown as using 36% less energy, would now be listed with a higher HERS value. The response I got back back from the rater was”
“That being said, the RESNET Board of Directors advised the SDC (Standards Development Committee) and ANSI Standards Committee to revise how the HERS Index is calculated. This change addressed many issues affecting the HERS Index. I’ve listed the two primary reasons for the changes below:
1. To address consistency among the various software providers
2. To implement calculating the HERS Index using an hourly algorithm instead of seasonal”
This did not really help me much since “To calculate a home’s HERS Index Score, a certified RESNET HERS Rater does an energy rating on your home and compares the data against a ‘reference home’ a designed-model home of the same size and shape as the actual home, so your score is always relative to the size, shape and type of house you live in.”
So the reference home should not have changed between the two runs. I hope someone can shine some light on this for me.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
What are the differences between your home and the reference home?
Ex. Room layout, windows, etc.
How do the new and old numbers compare to the reference home?
I'm not fully understanding your question.
They performed a HERS rating, using a reference home they input. To my basic understanding that should represent a model of a home that has the same wall area/cuFt with some standard window percentage and insulation values. The details of the reference home are not listed on the report.
What I pointed out the incorrect insulation values of my actual home. They fixed that and re-ran the HERS report and gave me a "worse" rating, which I assume should have been against the same reference home as the original report, why would that have changed?
Its basically like doing a HERS rating on an existing home, then take that home and replace the windows with better ones and add more attic insulation and reduce the air leakage and then be told you made the house worse.
S Cook, in this response you only note insulation values. But in your original question you wrote, "several errors regarding insulation levels, wall construction, Makeup air, HVAC equipment etc." We can't advise you unless we understand what all of the changes included.
Okay, for some reason I thought you were comparing the HERS rating to a model home in the neighborhood.
Yes, the index is scored on a scale relative to your home using standard construction techniques. You can contact RESNET directly but there were two methods (Seasonal, Hourly) used to calculate the score. There was some talk about standardizing which method was required BUT the hourly calculation seems to be encouraged.
Basically the calculation method is why the score went up, but it's irrelevant as long as you're comparing scores using the same method. Also, some software does a better job at modeling certain aspects of construction (ie. insulation depth, or the impact of ventilation) vs others. Sounds like your rater used the appropriate calculation method and made the necessary corrections for deficiencies within the software used to arrive at the number.
"Basically the calculation method is why the score went up, but it's irrelevant as long as you're comparing scores using the same method."
I think you are saying you can't compare a seasonal vs hourly calculated HERS rating? I don't see anywhere on the HERS report that shows what method was used to do the calculation, I'm probably just missing it. Strange that they would have changed the calculation method inside the span of 2 weeks. In any case it seems like this can lead to confusion for everyone. To me its like changing the definition of a gallon in MPG from one MFG to another.
"in your original question you wrote, several errors regarding insulation levels, wall construction, Makeup air, HVAC equipment etc."
I don't think this really helps other than your curiosity, but the main deltas that I recall were:
- R13 walls vs R19 on the first floor and R19+5 on the bonus space
- 2x4 vs 2x6 with energy corners, insulated door/window headers, advanced framing around windows/doors.
- Exhaust only ventilation vs Panasonic FV10VE1 balanced ERV
- Electric .98 EF HWH vs Hybrid 3.45 EF HWH
- Exposed Ductwork in Attic 20% vs 0%
- Heating Gas Only 98.5 Vs Hybrid HP / Gas (Seer 17 / 98.5)
- 100% CFL vs 100% LED bulbs (we used 7W surface mounted LED's on the ceilings) In the final report use of LED's was listed as using 1.5 MMBtu greater than the CFL's were which was interesting.
So when those values were corrected, the HERS value went up, i.e. closer to the reference home then the original run, which by HERS definition means its a less efficient home then it was.
If I were to compare this to the rating to the "model home" (which I was not).. my house is calculated as using 56% less MMBtu but has a HERS value that is 2 points (or 25) better. Again I don't know if this is season vs hourly data, I'm just going off the certificate you get with your house.
Seems like these energy usage deltas are fairly large to result in what are basically the same HERS value for the same homes. Perhaps it is a metric that has been so heavily massaged that its not useful for its original intended purpose. Don't know, just the entire thing seems very odd to me.
You could try contacting RESNET.
"I don't think this really helps other than your curiosity"
I was just trying to help you identify things that might affect your HERS score, based on what you wrote/didn't write. I am not actually curious about your project and I don't know much about the HERS rating system.